A thermoelectric material converts a temperature difference (thermal energy) into electric energy, and conversion efficiency η thereof is expressed in an expression (1) below.
                    η        =                                            Δ              ⁢                                                          ⁢              T                        Th                    ⁢                                    M              -              1                                      M              +                              Tc                Th                                                                        (        1        )            
In the expression (1) above, Th represents a temperature on a high-temperature side, Tc represents a temperature on a low-temperature side, and ΔT represents a temperature difference between Th and Tc (=Th−Tc). M is given in an expression (2) below, using a dimensionless performance index ZT representing an index representing performance of a thermoelectric material. This dimensionless performance index ZT is a value obtained by multiplying a performance index Z by an absolute temperature T, and expressed in an expression (3) below.
                    M        =                              1            +            ZT                                              (        2        )                                ZT        =                                            S              2                        ⁢            σ            ⁢                                                  ⁢            T                    κ                                    (        3        )            
In the expression (3) above, S represents a Seebeck coefficient (V/K) of a thermoelectric material, σrepresents a conductivity (S/m) of a thermoelectric material, and κ represents a thermal conductivity (W/mK) of a thermoelectric material. Z has a dimension defined by a reciprocal of a temperature, and ZT obtained by multiplying this performance index Z by absolute temperature T has a dimensionless value.
Conversion efficiency η given in the expression (1) is a monotonically increasing function of dimensionless performance index ZT. Therefore, increase in dimensionless performance index ZT is a key for improvement in performance. Conventionally, however, dimensionless performance index ZT has remained around 1, and a result exceeding this has not been reported.
It has recently been known (for example, L. D. Hicks et al., PRB 47 (1993) 12727 (NPD 1) and L. D. Hicks et al., PRB 47 (1993) 16631 (NPD 2)) or demonstrated (for example, L. D. Hicks et al., PRB 53 (1996) R10493 (NPD 3)) that Seebeck coefficient S and thermal conductivity x can be controlled by lowering a dimension of carriers (free electrons or free holes) and increasing phonon scattering owing to quantum wells and quantum wires.
A thermoelectric material in which carriers have further been lowered in dimension by forming particles has been known (Japanese Patent Laying-Open No. 2002-76452 (PTD 1)). The thermoelectric material, however, may suffer from lowering in conductivity, because an insulator buries a gap between particles.
Furthermore, it has been reported as another example of a lower dimension of carriers (H. Takiguchi et al., JJAP 50 (2011) 041301 (NPD 4)) that by forming nanoparticles of SiGe in a thin film of silicon germanium gold (SiGeAu) by annealing the thin film, thermoelectric characteristics are improved as compared with bulk SiGe.